In telecommunications networks that were originally conceived and further developed to provide voice connections, and which should be called voice networks, there are known call-switching service features that ensure that important calls are not lost. Thus, these voice networks, such as land line telephone networks, usually provide the service feature “redirect if busy” for the case that another connection is currently active with the called subscriber, or “redirect if there is no answer” if the called subscriber does not pick up the call, as well as “immediate call redirect” if the called subscriber would like all incoming calls to be redirected to another subscriber connection. Here, the call can be redirected to another subscriber connection in the same voice network or to another voice network connected to the first. Other possible destinations to which to redirect calls are to a recorded message or to an answering machine.
These possibilities described basically arise if a subscriber is connected to a voice network via a data network. Such a data network is, for example, a local computer network such as a Local Area Network (LAN) or the Internet. These networks had originally been developed for pure data transmission containing no spoken language, and thus do not initially provide the service features listed above. They are based on packet switching of data. Since the data transmission rates of these data networks have in the meantime reached very high levels, which are fundamentally suitable for voice transmission, processes were developed to also transmit voice, i.e., telephone calls over these types of networks as well. Thus, only a single network is required and not two parallel networks, the data network and the voice network.
An example of this is the use of IP-based data networks for voice transmission according to the H.323 standard. According to this standard, the following components are provided, among others: terminals, gateway and gatekeepers. Terminals are terminal devices connected to the data network. However, it is always necessary in these data networks that the subscriber signs on or logs in using these terminal devices. A gateway serves to convert the data protocols between the data network and the voice network. A gatekeeper is responsible for administering and controlling the user services as well as the network capacities in the data network operated under the H.323 standard. In particular, the gatekeeper performs the addressing of an incoming call in the data network. The directory number (telephone number) is converted into an address in the data network. In the gatekeeper, the information then exists indicating whether a terminal has signed on under this address of the data network. Telephone calls can be transmitted in the Internet or in IP-based networks using the H.323 standard. If the terminal device of the subscriber is not signed on, the problem arises that an incoming call in the voice network that is switched over to the data network through a gateway can get lost. Not even an answering machine can solve this problem since it also would have to be signed on. It is thus desirable in this case as well to reach a call redirect system.